1. Field of the Invention
This invention relates in general to equipment for perforating earth formations in cased wells, and in particular to an improved perforating gun.
2. Description of the Prior Art
There are several devices commonly used in perforating wells in the oil and gas industry. All of these devices are lowered into the well on insulated conductor cable. One type, known as a casing gun, comprises a thick metal pipe with a number of threaded ports. Jet explosive charges, known as shaped charges, are placed in the pipe in alignment with the ports. Threaded plugs are then secured in the ports. When at the desired depth, the charges are initiated by fuse cord and a detonator. The jet pierces the threaded plug, well casing, cement, and travels into the earth formation. The gun is retrieved and reused with new plugs.
One disadvantage of casing guns is that they are expensive due to material and labor cost in manufacturing the gun. Cleaning and reloading is time consuming, and numerous parts must be replaced such as the plugs and O rings.
Another type, commonly called a scallop gun, comprises a metal tube of diameter sufficiently small to be lowered through tubing. Shaped charges are carried inside the tube. Concave scoops are located on the outer surface of the tube adjacent each charge. When detonated, the jet pierces the tube wall, the well casing, cement, and earth formation. The tube is then retrieved and discarded. The depressions prevent burrs from protruding too far, and also reduce the thickness that the jet has to pierce before entering the well casing.
One disadvantage of the scallop gun is that it must be relatively thin compared to casing guns in order to be run through tubing. The compressive forces caused by the detonation could cause the tube to rupture, preventing if from being retrieved from the well. In order to avoid rupturing due to the detonation, expensive, high strength, heat treated steel is used. Since the gun has to be discarded after a single use, this technique is relatively expensive.
Another disadvantage with both types is that only a single charge is located at a particular depth. Although there may be several charges to a vertical foot, all of the charges are spaced one above the other. As explained in detail in my prior U.S. Pat. No. 3,089,416, perforating can cause or induce fracturing. If the charges are closely spaced vertically, the tendency is for the fracture to be induced vertically. Vertical fractures in the formation can be detrimental since they may lead into water bearing zones, causing more water to be produced along with the oil. On the other hand, horizontal fractures are beneficial, since they increase formation exposure at the desired point of production.
In my prior patent, I disclose a cluster of four charges that are especially oriented to induce or cause horizontal fracturing. The shock waves from a pair of horizontal parallel jets are added to the shock waves from upper and lower converging jets to cause the desired fracturing effect. That device, however, utilizes an expendable, non-retrievable housing. The detonation of the charge disintegrates the gun housing. Pieces from the metal housings of the charges and the gun housing may migrate with the well flow, causing clogging of the production equipment. Also it can fire only a single cluster in one run. It is a special purpose gun to be used only prior to hydraulic fracturing for controlling the fracture.